Surprising stability for sodium-ion batteries

Material degradation is a limiting factor for lithium-ion rechargeable batteries. Among other causes, each cycle of charging and discharging places mechanical strain on the battery’s anode as the electrode (typically graphite or a metal alloy) swells or shrinks with the addition and removal of Li ions. One would expect sodium-ion batteries, in many ways similar to their Li cousins, to experience more severe electrode fractures—and degrade faster—because of the Na ion’s bigger size. Yet Jun Wang and colleagues at Brookhaven National Laboratory have found that tin anodes in Na-ion batteries are unexpectedly resilient over many charging cycles. The team developed a full-field x-ray transmission microscope that could deliver in situ, three-dimensional tomographic images with 50-nm resolution during the anode’s repeated sodiation and desodiation. In the first sodiation, as Na ions became alloyed with the Sn electrode, the anode volume tripled and the surface area quintupled (the figure illustrates size- dependent changes). But after that initial damage, subsequent desodiation and nine further charging–discharging cycles produced no further fractures: The expansion and shrinkage were reversible and produced a minimal decrease in charge capacity. Armed with those results, the researchers plan to examine other electrode materials to better understand their stability and device potential. (J. Wang et al., Nat. Commun. 6, 7496, 2015, doi:10.1038/ncomms8496 .)